This new admin web-UI should become the single best choice to deal with the
configuration. Even experts will value the checks for coherent configuration,
that come bundled with the new wizard-style configuration admin panel.
If you deal with more than an initial configuration, sooner or later you will
get surprised (or doomed) by some dependency or side-effect. Honestly, me too.

Settings are written back now, but saved to trac.ini only on last page.
For now I consider it a valuable feature to be able to test and eventually
revert settings by just reloading the previous configuration from file.
This has to be tested and user reception will decide too, if it could become
the more successful approach.

All other components, that altered the configuration before, do it only in
memory now to effectively prevent file corruption by too fast trac.ini
reload cycles - lessons learned from #10871.

Included some missed goodies and more learned from Jun Omae's early review to
improve for i18n.

Adding more content and some eye candy to last wizard page including a few
error messages and logic for tracking overall configuration readyness.

Moved JavaScript code to a separate file to deflate the main template a bit.
Moved translatable strings from JavaScript to unify i18n into Python code.
Collected all configuration previews for easier maintenance into one dict.
More code comments, further re-formatting for PEP8 and enhanced readability.

Thanks to Jun Omae's early review I've been able to steer clear of some
template issues, avoiding even more on message extraction for i18n.